The present invention relates generally to an information recording medium disc and method and apparatus for recording/reproducing information using optically readable disc, and more particularly to optically reading system using a disc having a recording layer from which prerecorded information can be erased so that information signals or data can repeatedly be recorded and reproduced.
Recently, as the desire of high-density recording/reproducing of information signal has become stronger and stronger in various technical fields, high-density recordings of an information signal using information recording medium of various types are proposed.
As a recording medium with which recording/reproducing of an information signal can be readily done and it is also easy to rerecord (rewrite) an information signal with recorded information signals being erased, is known a magnetic recording medium of general structure having a recording layer made of a ferromagnetic material. Recently, as a recording medium which is capable of recording, reproducing and re-recording is used an information recording disc formed by providing a recording layer of a material, such as (TeOx), which undergoes phase variation from amorphous state to crystal state or from crystal state to amorphous state by the application of electromagnetic wave, electric field or heat, or an information recording disc formed by depositing a recording layer of magnetic thin film (for instance a magnetic thin film made of a substance such as TbFe, GdTbFe, GdCo) with which thermomagnetic recording utilizing a polarity reversal caused at Curie-point temperatures or compensation temperatures of the film materials, is to be carried out, on a substrate. Such recording is advantageous in that recording density is much higher than that in conventional magnetic recording.
In order that high-density recording is satisfactorily performed, an interval (track pitch) between recording tracks has to be narrow. However, narrow track pitch makes it difficult to cause a recording transducer, a reproducing transducer, or a recording/reproducing tranducer to accurately follow the tracks on an information recording medium. To solve this problem, various attempts have been made for applying a tracking control means of various types. In the case that the recording density is further increased, the recording apparatus is required to have an extremely high precision in connection with mechanical accuracy to prevent recording tracks from overlapping each other on recording, and therefore such apparatus tends to be extremely expensive.
It was proposed as a countermeasure for resolving this problem, that a guide track is formed in advance in an information recording medium disc in an unerasable state, which guide track is used for generating a tracking reference siganl used in tracking on both or one of recording of an information signal on a recording layer of the information recording medium disc and on reproduction of the information signal from the recording layer, in the case of a disc like information recording medium arranged to perform recording/reproducing an information signal using a tiny light spot.
While constant linear velocity (CLV) recording/reproducing of an information signal has been carried out conventionally for achieving high-density recording of an information singal on an information recording medium disc, it was also proposed to record firmly (i.e. in unerasable manner) a signal like a rotation synchronization signal (or velocity signal) used for recording/reproducing at a constant linear velocity, at a portion of the above-mentioned guide track used for generating tracking reference signal.
One of the above-mentioned already prosposed information recording medium discs is disclosed in Japanese patent provisional publication 58-57643. Accoording to the proposed technique a recording layer is deposited or a substrate of an information recording medium disc in which a pilot signal for enabling the constant linear velcdity recording/reproducing has been recorded while pregrcoving is also done. Another already proposed information recording medium disc is disclosed in Japanese patent provisional publication 58-88838. According to this disc, recording is performed by making a row of pits on a recording layer by a main beam on a land portion between guide tracks used for generating the above-mentioned tracking reference signal with the guide track being scanned by a sub beam so as to perform tracking control.
In the former of the conventional already proposed information signal recording medium disc, there arises a problem that since the main information signal is recorded in the recording layer located at the pregrooved portions in which the pilot signal has been prerecorded, the pilot signal is mixed with the main information signal when the main information signal is reproduced. As a result, signal to noise ratio it deteriorated. In the latter of the conventional already proposed information signal recording medium disc, there is a risk that the signal within the guide tracks for generating tracking reference signal is mixed with the main information signal when the recording track interval (track pitch) is made small in an atempt of achieving high-density recording, and therefore improvement has been desired.